Lie face up with your arms overhead and your legs extended, resting on the floor.
Lift your hands and feet a few inches off of the floor into a hollow position, and keep your abs engaged by pulling your belly button into your spine and pressing your lower back into the floor.
Reach your left arm to your right leg, then lower them toward the floor to return to that starting hollow position (don't lower back down to the ground completely). Then, reach your right arm to your left leg before returning to your hollow position.